U.S. Pat. No. 5,531,126 describes a Coriolis massflow/density sensor which can be installed in a pipe by means of a connecting element at the inlet end and a connecting element at the outlet end and through which a fluid to be measured flows during operation, comprising:
a single straight measuring tube having a longitudinal axis and extending between and fixed to the connecting elements; PA1 a straight dummy tube extending parallel to the measuring tube and not traversed by the fluid; PA1 a nodal plate on an inlet side and a nodal plate on an outlet side, PA1 one of which fixes the inlet-end portion of the measuring tube to the corresponding end portion of the dummy tube, and PA1 the other of which fixes the outlet-end portion of the measuring tube to the corresponding end portion of the dummy tube, so that the measuring tube and the dummy tube are arranged side by side; PA1 a support tube having its ends fixed to the respective connecting elements and having a longitudinal axis of symmetry parallel to the longitudinal axis of the measuring tube; and PA1 means which act only on the dummy tube to excite the measuring tube into flexural vibrations whose frequency is not, however, identical with the resonance frequency of the measuring tube, with the measuring tube and the dummy tube vibrating in antiphase. PA1 a single straight measuring tube having a longitudinal axis, an inlet end, and an outlet end; PA1 a support fixed to the inlet end and the outlet end, PA1 a cantilever PA1 an excitation arrangement for constantly exciting the measuring tube in the second fundamental flexural mode PA1 a sensor for the motions of the measuring tube on an inlet side and a sensor for the motions of the measuring tube on an outlet side which are located between the middle of the measuring tube and the inlet end and outlet end, respectively, at the same distance therefrom. PA1 a first portion which acts on the measuring tube in the direction of the intersection of a longitudinal axis of symmetry of the cantilever and the longitudinal axis of the measuring tube with a first excitation force, and a PA1 second portion, which acts on an end of the cantilever remote from the measuring tube with a second excitation force directed opposite to the first excitation force. PA1 a single straight measuring tube having an inlet end and an outlet end; PA1 an inlet plate fixed at the inlet end and surrounding the measuring tube; PA1 and outlet plate fixed at the outlet end and surrounding the measuring tube; PA1 a first support plate fixed to the inlet plate and the outlet plate and extending parallel to a first generating line of the measuring tube; PA1 a second support plate fixed to the inlet plate and the outlet plate and extending parallel to a second generating line of the measuring tube diametrically opposite the first generating line; PA1 a cantilever PA1 a longitudinal bar located opposite the cantilever and fixed to the first and second support plates, said longitudinal bar acting as a counterbalance; PA1 an excitation arrangement PA1 a sensor for the motions of the measuring tube on an inlet side and a sensor for the motions of the measuring tube on an outlet side which are located between the middle of the measuring tube and the inlet end and outlet end, respectively, at the same distance therefrom. PA1 a first part of a first brake assembly based on the eddy-current principle is fixed to the front surface of the plate in an area in which the axis point through said plate; PA1 a first part of a second brake assembly based on the eddy-current principle is fixed to the back surface of the plate in an area in which the axis of torsional vibration has a possible piercing point through said plate; PA1 the first brake assembly comprises a second part which is attached to a first holder fixed at least to the first support plate; and PA1 the second brake assembly comprises a second portion which is attached to a second holder fixed at least to the first support plate.
This prior-art Coriolis mass flow/density sensor is mechanically balanced only in a narrow range of density values--approximately .+-.5% of a rated density--for a given dimensional design, i.e., only at these density values will forces originating from the vibrations of the measuring tube be practically not transmitted via the connecting elements to the pipe. The above range is extended by the excitation "beside" the resonance frequency, but substantially more excitation energy is required than for excitation at the resonance frequency. The less balanced the mass flow/density sensor is, the more such forces and vibrational energy will be transmitted to the pipe; thus, however, vibrational energy is lost and measuring inaccuracy increases.
This unbalance has a disturbing effect not only in case of temperature-induced changes in the density of one and the same fluid but also particularly during the measurement of different fluids flowing in the pipe at different times, for example one after another.